Process for recovering sulfur dioxide from a gas containing same

ABSTRACT

PROCESS FOR THE DECOMPOSITION OF A METAL BISULFITE WHICH IS A PRESURSOR OF SO2 TO REGENERATE SO2, ESPECIALLY FOR USE IN A SYSTEM INVOLVING REACTION BETWEEN SO2 IN A GAS WITH A SULFITE TO PRODUCE THE CORRESPONDING BISULFITE. EXAMPLES OF SULFITES INCLUDE POTASSIUM, CESIUM, AND RUBIDIUM SULFITES. IN THE PROCESS, BISULFITE CRYSTALS ARE SEPARATED FROM A SOLUTION CONTAINING THE SAME AND SO2 PARTIAL PRESSURE LOWERING MATERIALS AND SUBSEQUENTLY PLACED IN EITHER SOLUTION OR SLURRY FORM AND PASSED INTO A DECOMPOSITION ZONE WHEREIN THE BISULFITE IS HEATED TO DRIVE THE SO2 OUT OF THE SOLUTION CONVERTING THE BISULFITE TO THE SULFITE WHICH CAN BE RECYCLED TO ABSORB ADDITIONAL SO2, THE CONCENTRATION OF BISULFITE IN THE SOLUTION FED TO THE DECOMPOSITION ZONE IS DESIRABLY MAINTAINED ABOVE 35 TO 50%, AND PREFERABLY HIGHER, AND THE OPERATING CONDITIONS OF THE DECOMPOSITION ZONE ARE CONTROLLED TO PROVIDE A HIGH PARTIAL PRESSURE OF SULFUR DIOXIDE IN THE SOLUTION AND AN INERT GAS IS ADVANTAGEOUSLY EMPLOYED WHEREBY HIGH CONVERSIONS OF BISULFITE TO SULFITE CAN BE OBTAINED.

Aug. 29, 1972 J. D. TERRANA ETAL PROCESS FOR RECOVERING SULFUR DIOXIDEFROM A GAS CONTAINING SAME Original Filed Nov. 9, 1967 3 Sheets-Sheet lam Qmmb kima ATTORNEYS Au 29, 1972 J TERRANA ETAL 3,687,623

PROCESS FOR RECOVERING SULFUR DIOXIDE FROM A GAS CONTAINING SAMEOriginal Filed Nov. 9, 1967 3 Sheets-Sheet 2 E: f2 1 E r L 29 JZM VWKWMRL I l? 141W) at {5 A a 1/ A 70 l I I 1L 13L Z W INVENTOR$ newTfififififllfi A50 6. Nll-zhgig 26' W XZ W W ATTORNEYS M ATER 3Sheets-Sheet 5 J. D. TERRANA ETAL RECOVERING SULFUR DIOXIDE FROM A GASCONTAINING SAME PROCESS FOR awry/c a a/ mm? Aug. 29, 1972 Original FiledNov. 9, 1967 INVENTORS FHCK D. TRRANA A50 19. HIM-5f? ATTORNITYS UnitedStates Patent Int. Cl. C01b 17/60 US. Cl. 423-242 12 Claims ABSTRACT OFTHE DISCLOSURE Process for the decomposition of a metal bisulfite whichis a precursor of $0 to regenerate S0 especially for use in a systeminvolving reaction between S0 in a gas with a sulfite to produce thecorresponding bisulfite. Examples of sulfites include potassium, cesium,and rubidium sulfites. In the process, bisulfite crystals are separatedfrom a solution containing the same and SO -partial pressure loweringmaterials and subsequently placed in either solution or slurry form andpassed into a decomposition zone wherein the bisulfite is heated todrive the S0 out of the solution converting the bisulfite to the sulfitewhich can be recycled to absorb additional S0 The concentration ofbisulfite in the solution fed to the' decomposition zone is desirablymaintained above 35 to 50%, and preferably higher, and the operatingconditions of the decomposition zone are controlled to provide a highpartial pressure of sulfur dioxide in the solution and an inert gas isadvantageously employed whereby high conversions of bisulfite to sulfitecan be obtained.

This application is a continuation-in-part of applications Ser. No.594,431, filed Nov. 15, 1966, and application Ser. No. 616,682, filedFeb. 16, 1967. This application is also a continuation of applicationSer. No. 681,643, filed Nov. 9, 1967. All of these applications areabandoned.

This invention relates to a process for the decomposition of a saltwhich is a precursor for a gas, such as potassium bisulfite which is aprecursor for S0 and which is a solution with other salts which lowerthe partial pressure of the gas in the solution, and more particularly,to a system wherein crystals of the salt are first obtained and then aslurry or solution of the salt is decomposed under conditions involvingthe use of an inert gas to provide high conversions of the salt torelease the gas. In another aspect, the present invention involvesincreasing the concentration of the solution before introducing it to adecomposition zone.

In particular this invention can be advantageously integrated with asystem disclosed in application Ser. No. 616,682 of Jack D. Terrana andLeo A. Miller, filed Feb. 16, 1967 and incorporated herein by reference.This application discloses a process for recovering sulfur dioxide fromsulfur dioxide-containing gases which involves contacting the gas to betreated with an aqueous solution of potassium sulfite to absorb the S0and produce an aqueous solution of potassium bisulfite, a precursor ofsulfur dioxide. Other suitable sulfites are cesium and rubidium sulfitesalthough for simplicity, the disclosure hereinbelow will refer topotassium sulfite. The S0 precursor, e.g. bisulfite is separated fromthe resulting solution, e.g. by crystallization out of the solution inthe form of potassium pyrosulfite crystals which are advantageouslyheated, preferably in the presence of water, to release sulfur dioxideand provide potassium sulfite. The process is re- Patented Aug. 29, 1972generative since the potassium sulfite is advantageously returned to theinitial absorption step.

The release of sulfur dioxide from a precursor such as potassiumpyrosulfite depends upon its partial pressure under given conditions,e.g. when in association with or without other materials including saltssuch as potassium sulfate and potassium sulfite which are generallypresent in the resulting solution produced by reaction of the sulfurdioxide-containing stack gas with the aqueous solution of potassiumsulfite. The partial pressure of sulfur dioxide in its precursor in thepresence of such other materials, e.g. potassium salts, in the reactionproduct solution at its boiling point at standard pressure conditionscan 'be so low, for instance, that it cannot be released in a feasiblemanner. In sharp contrast to this, its partial pressure when thepotassium pyrosulfite is in relatively pure form, for instance, a puritygreater than about 98 wt. percent on a dry basis, is relatively high,for instance about 300 mm. of Hg. The purity of the potassiumpyrosulfite is increased in accordance with the present invention togenerally greater than about 65 wt. percent, preferably greater thanabout to wt. percent, before it is decomposed. The potassium bisulfitecan be advantageously separated or recovered in the form of potassiumpyrosulfite by crystallization from the SO -partial pressure loweringmaterials in the solution and separation, e.g. filtration. In accordancewith another advantage of this invention the purity of the potassiumpyrosulfite is further enhanced by washing the crystals on the filterwith a solution formed by redissolving a portion of the potassiumpyrosulfite crystals.

The potassium pyrosulfite crystals removed by filtration are dissolvedor slurried in water before introduction into the decomposition zone toenhance their decomposition to produce sulfur dioxide. In accordancewith the present invention, the decomposition zone includes a strippercolumn to which the solution of potassium bisulfite is introduced andthrough which an inert gas is swept to remove the sulfur dioxide fromthe solution. The solution fed to the stripper column is preferablyheated since the partial pressure of sulfur dioxide in the solutionincreases with temperature, that is, up to the boiling point of thesolution, so that the partial pressure of sulfur dioxide in the vaporphase is less, e.g. substantially less than the partial pressure ofsulfur dioxide in the liquid phase, providing a pressure differentialfrom the liquid to the vapor. Temperatures of about 200 F., or 230 R, upto about 375 F. are suitable for use in the decomposition zone. Theinert gas, which may be any gas inert to the reactants, such asnitrogen, methane, argon, helium, etc., provides a driving force toremove the sulfur dioxide from the liquid into the vapor phase. In thissystem, although some water vapor will be removed with the sulfurdioxide, most of the water vapor will be condensed within the tower atthe operating conditions of the tower.

Furthermore, since the partial pressure of sulfur dioxide increases withincreasing concentration of solids, i.e. potassium bisulfite, it isdesirable that the solids concentration of the solution fed to thedecomposition be maintained high. The desired concentration, is,however, to be balanced against the advantages of using an aqueoussolution of potassium bisulfite in the decomposition zone since, inaddition to providing ease of handling, the use of aqueous solutionspermits the use of lower decomposition temperatures. Generally, asuflicient amount of water is added to the potassium pyrosulfitecrystals to form a pumpable slurry, e.g. 60 to 70% solids, and aboveabout 35 to 55%, preferably 40 to 50% solids are present when a solutionis fed to the decomposition zone.

The present invention may be better illustrated by reference to theattached drawings and the following example, which are not to beconsidered as limiting the invention.

FIG. 1 is a flow sheet of a system for the recovery of S which systemincorporates the present invention in an advantageous manner;

FIG. 2 is an enlarged schematic view of the absorption column 10 in thesystem depicted in FIG. 1;

FIG. 3 is a sectional view of a unit for removing particulate solid orliquid soluble contaminants from a gas stream; and

FIG. 4 is an elevational view of a packing column support tray.

Referring now to FIGS. 1 and 2, a gas stream containing sulfur dioxide,e.g., flue gases from a power plant or waste gas from an industrialplant, e.g. a sulfuric acid plant, or gas containing particulate solidsfrom an aluminum plant, is introduced into reactor 10, having an insidediameter of 30 inches, constructed of 304 type stainless steel andhaving any suitable corrosion resistant material liner 9 which isresistant against weak sulfuric acid, e.g. lead or suitable syntheticresin liners. The gas passes through line 12, having an internaldiameter of twelve inches. The reactor is of a length of 21 feet fromline BB. The amounts and rates given throughout this specification forgas and liquid specifications are based upon a reactor having thesedimensions for internal diameter, i.e. 30 inches or a cross-sectionalarea of about 4.9 square feet, and distances between componentspositioned in the reactor. It will be obvious to those skilled in theart that the amounts and rates of gases and liquids used will varydepending upon the size of reactor used and the distances betweencomponents employed in the reactor, however, these can be referred to asequivalents within the scope of the invention.

The concentration of sulfur dioxide in such gases is generally fromabout 0.001 to less than about mole percent and frequently is less thanabout 0.5 mole percent (about 1 percent by weight). For example, amodern electric power plant of 1,350,000 kw. capacity will burn about15,000 tons of coal per day. Much coal contains about 3.5 weight percentsulfur, or even more. The emission of sulfur dioxide from a plant ofthis size using such coal would then amount to about 1,000 tons per day,although the concentration of sulfur dioxide in the stack gases can bevery low, for instance on the order of 0.2 to 0.3 mole percent,depending upon the sulfur content of the coal. Significant amounts, forinstance greater than about 75 weight percent, of sulfur dioxide can beremoved from such gases.

The waste gases may also include S0 however the concentration of 80;,will vary according to the source of the gases and even flametemperatures used in processing, among other factors. It will usuallynot be higher than about 0.01 mol percent of the gas, normally beingwithin the range of about 0.0001 to 0.01 mol percent, usually themajority, e.g. more than about 70 percent, of the 80;, in the gas ispresent as S0 adsorbed on the surface of particulate solids. Aparticular advantageous feature provided by the prescrubber resides inits capacity to remove minimal amounts, e.g. from about 0.00001 to lessthan about 0.005 mol percent, of liquid soluble components in the gasstream; this being particularly useful when the gas is treatedpreliminarily by prescalping as described below and the residual amountsof 80;, in the gas to be treated in the prescrubber are minimal.

The gas stream in line 12 is generally at a temperature of up to about800 F., often about 150 to 600 F. For example, suitable gas streams foruse in this process include flue gases from power plants burning coal oroil which typically have a temperature in the range of 250 or 300 to 360or 400 F., off gases from sulfuric acid plants which typically have atemperature in the range of 150 to 200 F. and off gases from smelterswhich can have a temperature of up to about 800 F. However, attemperatures above about 400 F. it may be desirable to cool the gas to atemperature from about 225 F. ;to 300 F., e.g. by quenching with ambientair, to avoid deleteriously affecting the reactor, reactor lining orprescrubber operation. If the gases are below about 150 B, it may bedesirable to heat the gas in line 12 to assist in removal of water fromthe solution in reactor 10. The flue gases will generally have arelative humidity up to about 10 percent, usually from about 1 to 7percent. Fan 28 in FIG. 1, which draws stripped gas from the reactor,can, if desired, be arranged to blow gas through the reactor. Thestripped gases removed from reactor 10 via stack 14 and the incominggases in line 12 can be passed in a countercurrent heat relationshipthrough a heat exchanger (not shown), if desired.

Sulfur dioxide is absorbed from the gas stream in zone 30-of reactor 10by absorption through reaction with an aqueous solution of potassiumsulfite to produce an aqueous soluion of potassium bisulfite, and thestripped gases are emitted through stack 14. Although this illustrationwill use potassium sulfite, the corresponding sulfites of cesium andrubidium can also be used.

The gases, e.g., flue gases in this illustration introduced through line12 pass under bonnet 13 which is arranged over the inlet opening of line12 into reactor 10 and which extends into the reactor 10 a sufficientdistance to prevent water or solution from passing in front of the inletopening of line 13. It advantageously prevents plugging of inlet line 12by components, e.g. fly ash, recovered from the gas in zone 20. Bonnet13 extends upwardly at a slight angle to cause liquid to run back to thewall of reactor 10 and around the inlet opening of line 12 which isflush with the wall of reactor 10. The angle of bonnet 13 is generallyfrom about 5 to 60 degrees, preferably about 10 to 45 degrees, e.g. 30degrees, and bonnet 13 has a diameter at least equal to the diameter ofthe inlet opening of line 12, e.g. a 12 inch internal diameter. The mostupwardly projecting portion of the bonnet in zone 20 is located at adistance sufficiently away from nozzle 25 to avoid destroying the spraypattern but close enough to avoid excessive vaporization of the mistfrom the spray and droplets falling ofl? of the target and this distanceis generally from about 5 to 70 inches, advantageously from about 10 to30 inches, e.g. 15 inches, in the arrangements described herein. Anyliquid running down the wall of reactor 10 or striking the upwardsurface of bonnet 13 thereby is directed away from the inlet opening andto the wall of reactor 10 without passing directly in front of the inletopening.

Reactor 10, although advantageously shown as a single vessel, includesthree zones which may, if desired, be separate units. These zones are aprescrubber zone 20, an absorption or reaction zone 30, and a moistureentrainment or demister zone 40.

The waste gases entering reactor 10 first pass through prescrubber zone20 where particulate solid components, e.g. fly ash, and water-solublecomponents, e.g. S0 hydrocarbons including methane, ethane, propane,etc., in the gas are selectively removed from the S0 containing gas, theS0 in the gas in turn being selectively removed from the gas in zone 30.Since a majority of the in the gas is usually adsorbed on the surface ofthe solids in the gas, it is generally removed upon removal of the solidparticles. The solids, e.g. dust, in the gases are generally unreactedmaterials, e.g. fly ash produced by the chemical plant, ornoncombustible components of the fuel.

The chemical make-up of fly ash varies, of course, with the particularfuel being burned, but usually it is composed to a large extent ofsilica, alumina and iron, with other metal oxides such as oxides ofmanganese and vanadium frequently being present in minor amounts. Othersuspended particulate solids which may be present in the waste gasesinclude, for instance, particulate hollow carbon spheres which areparticularly found in oil-burner ofi-gases, and which, like fiy ash,contain significant quantities of adsorbed S0 The particle size of thesuspended solids found in waste gases can be within about 0.5 to

50 microns, but may be much larger, however the predominant number ofthe particles range up to about 10 microns.

For instance, the fly ash in a flue gas produced from a coal plant usinga Turbo feed coal grinding method had the following size distribution inthe flue gas.

Weight percent Size in microns: of total fly ash and from a coal plantusing a Cyclone feed coal grinding method had the following sizedistribution in the flue gas:

Weight percent Size in microns: of total fly ash The solids loading ingases can range from about 0.001 to 60 grains per cubic foot of gas, forinstance, and for flue gases will generally range from about 1 to grainsper cubic foot. If the solids loading of the gas is very high, e.g.generally containing more than about 1, e.g. from about 1 to 10 grainsof solid per cubic foot of power plant flue gas, which usually haveabout 1.2 to 4 grains of solid per cubic foot, and particularly if itcontains large particles, e.g. having its longest dimension greater thanabout inch, it may be desirable to prescalp the gas, for instance in anelectrical precipitator or Cyclone separator, see FIG. 3, beforeintroducing the gas in line 12. Generally from about 80 to 85 percent ofthe particulate solids can be removed by prescalping with a Cycloneseparator and a greater amount with an electrical precipitator, whichhowever, is more expensive than the separator.

While the interposition of devices such as electric precipitators andcentrifuges, e.g., dry cyclones, between the source of the waste gasesand the chemical absorption zone will readily serve to preliminarilyremove the bulk of the larger of such solid contaminants, in order toprovide essentially complete removal of these solids prior to contactingthe gases with the absorbing solution it has heretofore been necessaryto employ a great number of such devices in series or parallelrelationship. This results in high cost of operation or capitalinvestment. A particular advantageous feature provided by theprescrubber resides in its capacity to substantially remove even minimalamounts, e.g. from 0.001 up to about 1 grain per cubic foot of gas, ofparticulate solids which are not removed by the use of a separator orelectrical precipitator and unless removed, would deleteriously affectthe system; moreover, the solids are removed efiiciently and withminimum capital cost.

Scrubbing water is introduced via line 24 to the lower section ofreactor 10 and is upwardly discharged in the form of a fine spraythrough spray nozzle 25 having an orifice generally sufficient toprovide 0.1 gallon per minute (g.p.m.) per 2000 cubic feet per minute(c.f.m.) of gas and producing spray in an are generally from about 10 to125, e.g. 75 having spray droplets ranging in size generally from about200 to 800 microns, advantageously for removing particulates of a sizeup to about 60 microns. The temperature of the scrubbing liquid, e.g.water, is generally from about 50 to 120 F., preferably from about 70 to90 F. A plurality of nozzles can also be employed as shown in FIG. 2.Positioned generally from about 4 to 18 inches, e.g. about 9 inches,above the nozzle 25 is a substantially horizontally disposed, fluid,e.g. gas or liquid-permeable impingement target assembly having littlecontinuous extensive surface. It advantageously consists of columns ortabular members forming a lattice-like tray support 15, preferablyformed from an expanded metal, e.g. stainless steel. Held by the support15 is packing material 22 composed of about three layers of randomlyarranged, nonporous, packing components, e.g. Raschig rings, Interlox orBerl saddles, preferably of ceramic composition, the individualcomponents having a longest dimension generally from about 0.25 to 3.5inches, e.g. 1 /2 inches, and forming a packing column generally fromabout 1.5 to 5.5 inches in height and advantageously from about 3 to 4inches in height, weighing generally from about 30 to 70 pounds percubic foot and having generally from about 50 to percent free space.

As indicated in the drawing, the spray nozzle 25 is positioned nearenough to the support 15 to provide a continuous spray of water at asuperficial velocity sufficent to overcome gravity, contact and wet thetarget surface, generally from about 2 to 20 f.p.s., e.g. about 5 f.p.s.The water reaches, contacts and wets the column support and the layer ofpacking material retained thereon, and falls downwardly as largedroplets to carry away particulate solid and S0 components which areremoved from the gas stream. The spray of water entraps the larger solidparticles causing them to fall out of the gas stream while partiallydissolving water-soluble S0 in the gas stream and, as the waste gaspasses through the impingement material, substantially all, e.g. atleast about 90 percent, of the remaining smaller solids and S0 in thegas strike and stick to the wetted areas of the impingement material,forming droplets which enlarge to a point to overcome the force ofgravity and adhesion to the surface of the material and fall or arewashed off of the impingement material by the spray of water. Generally,in excess of weight percent, for instance in excess of 99 percent, ofthe particulate solids may be removed from the gas. The removal of S0 isparticularly desirable since in this system it produces potassiumsulfate which deleteriously affects the recovery of $0 by, for instanceby consuming potassium sulfite which would otherwise be consumed by SO2-Waste gases entering the column via line 12 proceed upwardly through thesupport 15 and packing 22. The gas is advantageously contacted with theliquid by conducting the gas and liquid concurrently to a fluid, e.g.gas or liquid-permeable impingement target; the gas being conducted tothe target at a superficial velocity suflicient at the time of contactwith the target to permeate and pass through the target in the presenceof a liquid being conducted to the target at a superficial velocitysufficient at the time of contact with the target to wet the target andinsufficient to permeate the target in substantial amounts. Generally,the velocity of the gas to the target is sufficient to have apredominant amount, for instance at least 60 volume percent andpreferably from about 90 to 100 volume percent, permeate and passthrough the target. Generally, the superficial velocity of the gas atthe time of contact with the target will range from about 2 to 15 f.p.s.and preferably from about 7 to 12 f.p.s., e.g. l0 f.p.s.

The superficial velocity of the liquid at the time of contact withtarget is generally from about 2 to 15 f.p.s. and preferably from about5 to 10 f.p.s., e.g. 8 f.p.s. The amount of liquid employed issufficient to effect transfer of particulate solid or liquid-solublecomponents in the gas to the liquid on contact with the target and thiswill depend upon the amount of particulate solid or liquidsolublecomponents in the gas, generally the amount of liquid will range fromabout 0.01 to 1 g.p.m. per 2000 c.f.m. of gas containing from about 0.5to 30 grains per cubic foot of particulate solid components andsuflicient to dissolve the liquid-soluble components in the gas. Thepressure drop of the gas through packing 22 will generally range fromabout 0.25 to 0.5, e.g. 0.4, inch of water at 2000 c.f.m. of gas.Moreover, substantial interstitial holdups of liquid in the packing isavoided.

The liquid employed in the present invention can be any suitable liquidwhich is chemically inert to the impingement target and which does notdeleteriously affect the mass transfer, particularly when the masstransfer of non-solid components is involved, the liquid isadvantageously one that will also selectively dissolve and remove thenon-solid components. For instance, water can be employed to selectivelyand simultaneously re move particulate solids and S from a gas stream.Suspended solids present in the gases impinge upon and are restrained bythe wet surface presented by both the sup port 15 and the layer ofpacking material 22. These solids are washed off the packing and supportby the downwardly falling, spent scrubbing water which has been checkedand driven back by the target assembly. The spent scrubbing water,containing removed solids and absorbed sulfur trioxide, flows by forceof gravity down the funnel shaped sides of collection bottom 17 ofcolumn and out the spent scrubbing water discharge line 26. An importantfeature is the positioning of a surface such as collection bottom 17 ina droplet receiving relationship with the impingement target to receivedroplets falling off of the target and remove them from zone 20 beforesubstantial evaporation of the droplets can occur and thus substantiallypreclude the return of particulate solid, e.g. fly ash, orliquid-soluble, e.g. S0 components to the target by the incoming gases.Referring to FIG. 2 the distance between points a-a in this illustrationis 6 feet and surface 17 projects downwardly at a 60 angle to outlet 26.Suspended solids and sulfur trioxide are thus removed from the wastegases before the latter come in contact with the chemical absorbingsolution. If desired, the spent scrubbing water, which may often have apH of about 2 to 4, depending on the amount of sulfur trioxide in thewaste gases, may be treated for separation and recovery of the solids,e.g., fly ash, and the sulfuric acid. One highly advantageous feature ofthis prescrubbing arrangement is that low volumes of water can be usedto wet the impingement target, a particularly attractive feature sincethe amount of water within the present system can affect the overallefficiency of the process. Accordingly, it is desirable to only use anamount of water sufiicient to contact, wet, and fall from theimpingement target surface and collect the particulate solid materialsand S0 Generally, less than about 0.1 g.p.m. of water, preferably fromabout 0.01 to 0.07 g.p.m., e.g. 0.05 g.p.m., per 2,000 c.f.m. of gases,are used. The advantage of using less than about 0.1 g.p.m. isillustrated in Table I below. By using this prescrubber arrangement, itis possible to control any increase in humidity of the gases passedthrough prescrubber 20 generally to not over about 8 to 10%, preferably2%, and the temperature drop across the prescrubber is generally lessthan about 60 F., preferably less than about 50 F.

Use of the scrubbing apparatus described here and in the drawings,particularly in FIG. 2, has been found to provide, in addition to anextremely low temperature drop and minimal increase in the relativehumidity of the gas across the scrubbing zone, particularly whenprocessing gases at temperatures up to 500 F., also provides an assemblywhich is surprisingly free of plugging difiiculties in contrast to aprescrubbing assembly employing continuous overhead water spray deliverywhich resulted in a significant temperature drop, e.g. about 100 F and asignificant increase in relative humidity, e.g. about 10 percent, andalso became clogged within a short time, e.g. 2 hours, with solidsremoved from the gas stream. The positioning of the spray nozzles on theunderside of the target permits extended operation times, e.g. up toabout 16 hours, without such plugging problems. If desired, however,there may also be provided in the scrubbing apparatus of the presentinvention a second spray asrently flowing gas and liquid streams,although it may be activated periodically by actuating device 29, apressure drop control, for instance about every 8 to 16 hours for a 1 to3 minute period, to direct a downward spray on the target and thusirrigate the packing material and provide for continuous operation.

The absorber or reaction zone 30, as shown, is advan tageously designedfor intimate contact of countercurrently fiowng gas and liquid streams,although it may be designed for concurrent fiow if desired. As shown,the absorber section is illustrated with two substantially horizontallydisposed sieve trays, e.g. which can be of a conventional type. Bubblecap trays can also be used. The gases are passed through reactor 10 at asuperficial velocity sufiicient to maintain liquid on the contact traystherein but not so great as to blow liquid out of the reactor. Typicalaverage superficial velocities of the gases through absorber section 30of reactor 10 are generally at least about 1.5 feet per second (f.p.s.)and advantageously from about 2.0 to 8.5 fps. The potassium sulfitesolution is introduced into reactor 10 through line 16, generally at arate of from about 0.1 or 0.4 to 20 g.p.m., preferably from about 2 to 8g.p.m., for each 2000 c.f.m. of gas, and the potassium bisulfitesolution is removed through line 18. Potassium sulfite, generally fromabout 40 to 60 weight percent, e.g. 50 percent, of the total solution tobe introduced, is conducted from line 16 through line 16" and line 44'of demister 40, described below, to fall onto the surface of tray 32 andflow from tray 32 onto tray 34. Additional solution can be added, ifdesired, through line 16. The potassium sulfite reacts with the sulfurdioxide in the gases passing through the sieve trays to produce anaqueous solution of potassium bisulfite which passes from tray 34 intothe catch basin formed by downwardly projecting bafile 36 from which itis removed through line 18. The sulfur dioxide content of the gas issubstantialy reduced, for instance, to less than about 0.02 mole percentin a stack gas containing more than about 0.2 mole percent. Thepotassium bisulfite is separated, e.g. crystallized, and can berecovered in crystalline form as potassium pyrosulfite which issubsequently decomposed to produce potassium sulfite and sulfur dioxide.Potassium bisulfite is transformed to potassium pyrosulfite duringcrystallization. The sulfur dioxide is drawn off and can be eithercooled and compressed to provide a liquid product or sent as a gas to asulfuric acid plant or sent to a reduction furnace for conversion toelemental sulfur. The potassium sulfite can be recycled to the reactionzone wherein additional sulfur dioxide is absorbed. The reactionsutilized include:

heat KzSzO5 K2503 502(8) H20 aqueous For Reaction I to proceed, thetemperature of the solution in absorption or reaction zone 30 should bemaintained above the temperature at which sulfur dioxide is absorbed byreaction with the aqueous solution of potassium sulfite, and below thetemperature at which potassium bisulfite decomposes or Reaction IIIproceeds, e.g. below about 230 F. In general, the cooler the solution ofpotassium sulfite, the more readily sulfur dioxide will be absorbed bythe solution and react with potassium sulfite. With stack or furnacegases, however, the tem pcrature of the solution will generally be aboveabout F. or F., although ambient temperatures are suitable. Preferably,the temperature is maintained below about 190 F., e.g. at about to or*F., since above these temperature ranges Reaction I begins to slow to apoint where sulfur dioxide will not be readily absorbed into solutionbecause the partial pressure of the sulfur dioxide becomes too high.Since the stripped fiue gas is ultimately, after processing in demisterzone 40, discharged into a stack, it is desirable to maintain thetemperature of the stripped gases at a temperature sufficiently high tomaintain their buoyancy so that they will rise in the stack, e.g., aboveabout 185 F. At lower temperatures, e.g. 135 R, a fan can be used todraw oif the gas. Generally the temperature of the gas from the absorberzone 30 into demister zone 40 ranges from about 120 to 180 F., e.g. 135F. The temperature of the stripped gas in line 14 out of the demistergenerally ranges from about 110 to 170 F., e.g. 130 F., but it can beincreased, if desired, by the introduction of a hot furnace gas throughline 14. Generally the gas out of the demister is from about 85 to 100%saturated with water.

Potassium sulfite solution is also advantageously sprayed against theunderside of trays 32 and 34 by spray elements 38. Elements 38 arelocated generally from about 4 to 18 inches, e.g. 9 inches, from theirrespective trays. The velocity of the liquid is sufiicient to overcomegravity. As the potassium bisulfite solution is formed on the trays,water is stripped from the solution by the hot flue gases which tends tosupersaturate the solution and crystallize potassium bisulfite and plugthe trays. Potassium sulfite solution from line 16 is passed to thespray elements through line 16". The sprays are directed against thebottom of the trays where the hot flue gases first contact the trays,since at this point most evaporaion of water occurs and, therefore,crystallization. A suficient amount of solution is sprayed upwardlyagainst the under or contact surface of the trays to keep the solidsdissolved in the solution on the trays, particularly at the surfacewhere the flue gases impinge and thus advantageously provide forcontinuous operation of the process. It is desired to keep the amount ofsolution passed to the spray elements as low as possible and generallythe amount of solution is less than about 0.1, or 0.4, g.p.m. per 2000c.f.m. The higher the temperature of the waste gases, the higher therate of solution required to be fed to spray elements 38. For example,with gases entering reactor 10 at 300 F., about 0.1 to 0.2 g.p.m. per2,000 c.f.m. of gas is suitable.

The potassium sulfite solution is passed through the absorber section 30in an amount suflicient to react with the sulfur dioxide in the fluegas, i.e., absorb the sulfur dioxide in the solution, and producepotassium bisulfite. Generally these are stoichiometric amounts andadvantageously a solution containing about 25 weight percent K 80 and 25weight percent KHSO can be employed.

The solution of potassium sulfite introduced into the reaction zone ispreferably a recycle stream and, generally, contains from about 30 or 40to 75, preferably about 40 or 50 to 65 weight percent solids of whichgenerally at least about 50 percent, desirably above about 75 percent,is potassium sulfite and the balance is essentially potassium bisulfitewith, possibly, some sulfate. This recycle stream is preferably asaturated solution of potassium sulfite. The potassium sulfite solutioncan contain a sufficient amount of an oxidation inhibitor, for instancehydroquinone, e.g., about 0.001 to 0.1 percent, to inhibit the oxidationof the sulfite ion. The temperature of this stream is controlled toavoid upsetting the requirements of reactor 10. The temperature of therecycle stream is typically from about 90 to 160 F. In general, asuflicient amount of potassium sulfite solution is contacted with thewaste gas in reactor 10 to remove as much of the sulfur dioxide aspossible, desirably above about 90 to 95 percent, and the residence timeof the solution in the reactor, etc., is adjusted accordingly. Thesolution flow rate in absorber zone 30 is normally maintainedsufiiciently fast, and the residence time of the solution in absorberzone short enough, that crystallization problems do not occur within theabsorber zone. With a slow flow rate and a high residence time,absorption of sulfur dioxide will occur; however, the solution becomesvery concentrated and, therefore, the danger of crystals depositing fromthe solution and plugging the sieve trays is increased. The flow rate ofthe solution will depend upon the temperature of the gas, the amount ofS in the gas, the

temperature and concentration of the potassium sulfite solution, itwill, however, generally range from about 0.1 to 20 g.p.m., preferablyfrom about 2 to 8 g.p.m. per 2000 c.f.m. of gas. Bafiie 36 forms acollector trough or catch basin for the potassium bisulfite solution.Baffle 36 can be sloped, e.g. from about 10 to 45, or horizontal, sothat any crystals contained in the potassium bisulfite solution willflow downwardly toward line 18. A relatively thick layer, e.g. a fewinches (e.g. 8 inches) or several feet, of solution is maintained onbaflle 36 to avoid formation of crystals in the solution and,preferably, the hold-up time in the catch basin is short, for instancegenerally from about 5 to 10 minutes.

The product of the reaction zone is preferably a saturated solution ofpotassium bisulfite, and, accordingly, the concentration of the solutionis desirably maintained at just below saturation by the addition ofsuflicient water to avoid precipitation of potassium bisulfite. As notedabove, this would occur as the crystallization of potassium pyrosulfite.

The solution resulting from a reaction of an aqueous solution ofpotassium sulfite and a stack gas inherently contains many ingredients,for instance the following is an example:

The sulfur dioxide is present in chemically combined form, for instanceas potassium bisulfite, which can be considered a sulfur dioxideprecursor, in the solution and is present or in contact with SO -partialpressure lowering materials for instance, metal salts, e.g. alkali metalsalts, generally the potassium salts such as unreacted potassium sulfiteand potassium sulfate, produced by reaction be tween sulfur trioxide andpotassium sulfite. The purity of the potassium bisulfite in contact withthe SO -partial pressure lowering materials is generally less than about60 wt. percent on a dry basis.

The amount of solids in the solution will vary depending upon thetemperature, but it is generally maintained sufficiently high forefficient sulfur dioxide recovery. There will generally be between about40 or 45 and 75 weight percent, preferably between about 45 or 50 and 55or 65 weight percent, solids in the solution. The amount of potassiumbisulfite and potassium sulfite in the solids will vary depending uponthe total percentage of solids and the temperature of the solution. Ingeneral, about 5 to 50 or 60 weight percent, preferably 10 to 35 or 50weight percent, is potassium bisulfite and generally 40 or 50 to 95weight percent, usually about 50 or 65 to weight percent, is potassiumsulfite. For example, at 77 F. a saturated aqueous solution of potassiumpyrosulfite and potassium sulfite will contain about 5% potassiumpyrosulfite and 47% potassium sulfite; at about 104 F. this solutionwill contain about 7% pyrosulfite and 48% sulfite; and at about 149 F.the solution contains about 12% pyrosulfite and 48% sulfite.

The stripped flue gases in reactor 10 pass from absorption reaction zone30 to the moisture entrainment or demister zone 40 which includes awoven mesh contact area. The pressure drop of the gas through zone 30will generally range from about 1.5 to 4.5, e.g. 2.5, inches of water at2000 c.f.m. of gas. The woven mesh 42 is a material chemically inert tothe components of the system, e.g. 304 stainless steel, and similar instructure and appearance to steel wool; it is shown in FIG. 2 as beingretained on a column support 43. Potassium sulfite solution from line 16passes through line 16", and lines 44 and 46' to nozzles 44 and 46,respectively, generally positioned from about 4 to 18 inches, e.g. 9inches, from 42, which continuously spray the solution onto the wovenmesh 42 from opposite sides of the assembly, i.e., top and bottom, toadvantageously avoid plugging problems and maintain a continuousoperation. The demister zone 40 serves to remove droplets of solutionfrom the gases exiting from reactor 10 to thereby limit chemical losses.The droplet can be liquid or solid forms of the reaction productgenerally small enough, for instance from about 1 to 100 microns insize, such that it can be supported in the velocity of the rising gasflow, normally between about 2 to 7 fps. It was surprisingly found thatwhen the contact material referred to was of a Woven mesh-likestructure, small amounts of solution could be advantageously employed.However, a material providing a large amount of contact surface and ahighly irrigatable contact surface, e.g. Raschig rings, can be employedbut require larger amounts, e.g. 15 g.p.rn. of solution. The thicknessof demister zone 40, e.g. about 1 to 4 inches, is sufiicient to removedroplets from the exit gases but not so large as to create a largepressure drop, and the gas flow through demister zone 40 is below thepoint at which droplets from the contact area would be re-entrained. Themaximum desirable pressure drop'is generally in the range of about A to/2 inch of water and, typically, gas velocities of about 2 to 6 f.p.s.are suitable. Generally, solution is added to the demister zone in anamount sufiicient to avoid plugging of the demister and insufficient forentrainment in the exiting gases. This demister embodiment is highlyeflicient and only requires the use of small amounts of solution. Forexample, generally about 2 to or 6, preferably about 3 to 3 /2, or 4,g.p.m. of solution per 2,000 c.f.m. of gas flow are suitable, beingdivided preferably equally between nozzles 44 and 46. The amount ofsolution used will decrease with decreasing concentration of thesolution and it is desirable to maintain the flow as low as possible.The dernister zone 40 also functions to remove the last traces of sulfurdioxide contained in the exiting gases. For example, the demisternormally removes an additional 1 to 2% of the total sulfur dioxide inthe incoming waste gases.

In this system 'sufiicient amounts of the SO' -partial pressure loweringmaterials are separated from the potassium bisulfite to increase thepartial pressure of the sulfur dioxide in the potassium bisulfite. Theseparation of the SO -partial pressure lowering materials can beeffected by any suitable procedure, by for instance by selectivelyextracting the potassium bisulfite from the solution or by extractingany one of the sO -partial pressure lowering materials from thesolution. The separation is preferably conducted by crystallizing thepotassium bisulfite (which transforms to potassium pyrosulfite duringcrystallization) out of the solution for further treatment in accordancewith the system as set forth below.

The release of sulfur dioxide from a precursor such as potassiumpyrosulfite depends upon its partial pressure under given conditions,e.g. when in association with or without other materials including saltssuch as potassium sulfate and potassium sulfite which are generallypresent in the resulting solution produced by reaction of the sulfurdioxide-containing stack gas with the aqueous solution of potassiumsulfite. The partial pressure of sulfur dioxide in its precursor in thepresence of other materials, e.g. potassium salts, in the reactionproduct solution at its boiling point at standard pressure conditions isso low, for instance for the typical solution described in an examplebelow it is about 1.5 mm. of Hg, that it can not be released in aneconomically feasible manner. In sharp contrast to this, its partialpressure when the potassium pyrosulfite is in relatively pure form, forinstance a purity greater than about 98 wt. percent on a dry basis, isrelatively high, for instance about 300 mm. of Hg. The purity of thepotassium pyrosulfite is increased in accordance with the presentinvention to generally greater than about 65 wt. percent, preferablygreater than about 90 or 95 wt. percent. The following table sets forththe partial pressure of the sulfur dioxide in potassium pyrosulfite forthe indicated purity.

TABLE Purity of K S O Partial pressure of S0 The potassium bisulfite canbe advantageously separated or recovered advantageously in the form ofpotassium pyrosulfite by crystallization from the SO artial pressurelowering materials in the solution. Crystallization of potassiumpyrosulfite can be accomplished using suitable crystallizationprocedures, for instance by supersaturating the solution by heating itin a vacuum or advantageously by cooling the aqueous potassium bisulfitesolution to a temperature at which a substantial portion of thepyrosulfite crystallizes, e.g. below about 100 F. or 110 F., with thelower limit being dictated by economics. For example, when a saturatedsolution of potassium bisulfite at 149 F. is cooled to 104 F.,approximately 40% of the pyrosulfite crystallizes whereas when thesolution is cooled to 77 F., approximately 70% of the pyrosulfitecrystallizes. Since potassium sulfite is more soluble than potassiumbisulfite, substantially pure, e.g. greater than about wt. percent,pyrosulfite crystals can be obtained.

The potassium pyrosulfite crystals can be separated, e.g. by separationtechniques such as centrifugation or filtration and heated to thedecomposition temperature therefor and under ambient pressureconditions, these temperatures are generally greater than about F. andsutiicient to decompose the potassium pyrosulfite, for instance aboveabout 230 F. and up to about 600 F. but preferably below temperatures atwhich substantial amounts of potassium sulfate form, e.g. 400 'F., andpreferably above about 300 F. under essentially anhydrous conditions, torelease sulfur dioxide and convert the potassium pyrosulfite topotassium sulfite which is suitable for reuse. This decomposition methodeliminates the need to vaporize large amounts of water to remove sulfurdioxide when anhydrous conditions are desired. The hot potassium sulfiteproduced upon decomposition of the crystals is combined with thefiltrate resulting from the separation of the potassium pyrosulfite andrecycled to the reaction zone.

Referring to FIGS. 1 and 2 again, the potassium bisulfite solution isremoved from reactor 10 through line 18 to surge tank 48 from which itis pumped (e.g. generally having only a residence time of only about 5to 20 minutes in the surge tank) expeditiously to avoid substantialcooling and crystallization and plugging within the tank. It is pumpedthrough line 52 by pump 54 to the flash cooler-crystallizer zone 50.Line 18 has a vacuum breaker line 18 associated with it. The solutionpumped through line 52 may, if desired, be passed through a filter 56 toremove solids from the solution.

The flash cooler-crystallization zone 50 includes a flash chamber 58wherein the solution can be advantageously cooled by vaporization ofwater from the solution, and a crystallization tank 60. Flash chamber 58is maintained under vacuum, generally of about 0.5 to 4 p.s.i.a.,usually from about 0.8 to 1.5 p.s.i.a., by steam ejectors 62 and 64.Such ejectors can draw a vacuum of about 0.5 p.s.i.a. As shown, steamejectors 62 and 64 draw a vacuum on the chamber 58 through line 66. Thepressure in chamber 58 is controlled by the addition of air through line68 to control the temperature of the solution. The control of thepressure in turn controls the boiling point and pressure drop, andtherefore the temperature of the solution in the flash chamber and theamount of water vaporized from the solution. Heat exchanger 70 in line66 condenses the water evaporated from the solution in chamber 58. Thecondensate passes through line 72 for use in dissolving the potassiumpyrosulfite crystals. The steam from ejectors 62 and 64 is alsocondensed and passed through lines 74 and 76 for later use in thesystem.

The solution in chamber 58 is normally cooled to a temperaturesufiicient to crystallize, i.e., remove from solution, a sufiicientamount of potassium pyrosulfite to compensate for the sulfur dioxideabsorbed from the gas stream in line 12 and thereby maintain a properbalance of solids in the solution for recirculation thereof. Thepotassium bisulfite crystallizes as potassium pyrosulfite. The amount ofpotassium pyrosulfite crystals removed depends upon the conversion ofpotassium sulfite to bisulfite in the stripper and generally, from about3 to 15 pounds of potassium pyrosulfite crystals are removed for eachpound of sulfur dioxide absorbed from the Waste gases. For instance, ifthe conversion of sulfite to bisulfite in the aqueous solution is about50%, about 7 pounds of crystals will be removed in order to provide asolution particularly suitable for reuse in the absorber section,whereas at 20% conversion this amount will increase to about 15 poundsof crystals per pound of sulfur dioxide, and at a theoretical 100%conversion, the amount will be about 3 to 3 /2 pounds per pound ofsulfur dioxide. The temperature drop in chamber 58 is determined bycontrol of the inlet temperature of the solution, the pressure withinchamber 58, residence time of the solution in chamber 58 (e.g. generallyfrom about 15 minutes to 2 hours), recycle ratio, etc. Sincetemperatures of the inlet solution generally are in the range of about80 to 200 F., usually from about 130 F. to 160 (F. for power plant typegas processing and usually from about 80 F. to 110 F. for sulfuric acidplant type gas processing, and the solution will boil at about 104 F. at1 p.s.i.a. and about 85 F. at 0.5 p.s.i.a., temperature drops generallyof about 10 to 70 or 90 F., preferably about 30 to 50 F. or 70 F., areusually required to crystallize the desired amount of potassiumpyrosulfite. The amount of water removed in the flash cooler from thesystem is advantageously used to maintain the water in the system inbalance by correlation of the amount of water removed in the flashcooler with the amount of water removed by the gas stream in thereactor. Although water can be easily removed in the flash cooler, itrequires an input of energy whereas if removed in the reactor, theenergy used is only such as is normally present. Thus, a particularlyattractive feature provided by prescrubber 20 resides in accomplishingthe prescrubbing with liquid, e.g. water, while at the same timeavoiding a significant drop in the temperature of the gas and asignificant increase in the relative humidity of the gas; thus providingthe gas in the absorber zone at optimum temperatures and relativehumidity for absorbing water from the system inexpensively andefiiciently. In the event a gas stream having a low temperature and highrelative humidity is employed and has limited capacity for absorbingwater, great flexibility is provided for the system in that excess watercan be removed by the flash cooler.

The efiiciency of the flash cooler in removing water also provides foradvantageous control of filtration conditions. The filtration systememployed can be a simple rotary filter and advantageous control over thetemperature and residence time of the solution can be obtained withfacility to control the size of crystals to enhance filterability withincreased filtration rates and provide crystals of greater purity, e.g.99 percent pure. It also desupersatures without nucleation andsubstantially avoid plugging problems which would characterize the useof a heat exchanger under the conditions of processing in the presentsystem.

The cooled solution, or slurry, is passed from flash chamber 58 tocrystallization tank 60 through line 78. The solution residence timewithin the crystallization tank 60 is sufiicient to produce a propersize crystal for filtration, e.g., crystals larger than about 300 meshand up to about 10 mesh. Generally a residence time of from about 5minutes to 2 hours, or more, preferably of about 10 minutes to 45minutes are suitable for the production of crystals of the desired size.Crystals can be obtained wherein about percent of the crystals arebetween 10 and 60 mesh using a 45 minute retention time. Anotherimportant factor in producing crystals of this desired size is thecontrol of the pH of the solution, e.g. generally between about 6.6 to7.4. The pH can be controlled by adjusting the specific gravity of thereaction solution conducted out of the reactor, for instance usingdensity control device 69 in line 71. Recycle line 80 and pump 82 areprovided to control the temperature and residence time of the solution.The bottom of flash chamber 58 slopes and the line 78 extends to thebottom of tank 60 to avoid plugging by crystals formed in chamber 58 andtank 60. A demister 84, arranged in the upper section of flash chamber58, is constructed and operates in a manner similar to demister 40 toremove entrained droplets in the gas passing through line 66.

The slurry of potassium pyrosulfite crystals formed in crystallizationtank 60 is pumped by pump 86 through line 88 to a rotary drum vacuumfilter, generally designated as 90. The drum filter includes a rotarydrum 92 and a pan 94. The slurry is introduced into pan 94 through line88 and drawn by a vacuum applied internally of drum 92 through thefilter surface of the drum by vacuum pump 96. The filtrate solutiondrawn from filter drum 92 is passed through line 98 to filtrate tank 100where it is collected and from which it is subsequently pumped throughline 102 by pump 104 to absorber feed tank 106.

The potassium pyrosulfite crystals retained on the filter surface ondrum 92 are removed by a scraper or doctor blade 108 and passed to adissolver tank 110. The filtration rate of the filter is maintained ashigh as possible while still obtaining a clear filtrate or solutionleaving the filter through line 98. Filtration rates of about 900 to4,000, preferably 1,500 to 3,000, pounds of crystals per hour per squarefoot of surface area of the filter are usually obtained. The higher thefilter rate the smaller the surface area of the filter required. Thesize of the filter cloth used can be important in maintaining a clearfiltrate. For instance, if crystals filter through the cloth, the pH ofthe filtrate solution will drop, e.g. to a pH of about 6.3, and thusadversely affect the recycle stream by deterring the absorption of as aresult of an increase of the bisulfite. The vacuum applied to the drumfilter by pump 96 is maintained so that the vacuum applied at drum 92 isless than the vacuum, e.g. higher pressure, applied in chamber 58. Touse a lower pressure at this point in the system would additionally coolthe solution to crystallize additional potassium pyrosulfite inside drum92 which would plug the system. The filter screen utilized on drum 92 ispreferably a monofilament cloth, e.g., nylon, chemically inert to thesolution and slurry, which has openings generally of about 10 to 50, or60 microns in size, preferably about 20 to 40 microns in size. Thecrystals can be advantageously washed by spray 112, preferably using aside stream of the 50 percent solution being conducted to the strippercolumn, on the filter surface to further enhance the purification ofthem. Since the 50 percent potassium bisulfite solution would besaturated, substantially none of the crystals are dissolved and thepurity of the crystals are increased by displacement washing of themother liquor. Also, if desired, a steam sparge or blowback 114 can beapplied to the filter surface near scraper blade 108 to clean the filter15 surface by removing any crystals not removed by blade 108.

The potassium pyrosulfite crystals can be advantageously heated in thepresence of water to enhance their decomposition to produce sulfurdioxide at relatively low temperatures. The water employed can beresidual water of the mother liquor contained by the crystals or it canbe added water; it can be in any suitable form, e.g. liquid or vaporform; and it is used in amounts sufiicient to enhance the decompositionof the potassium pyrosulfite to produce sulfur dioxide. These amountsare at least about 0.01 weight percent, generally from about 1 to 99weight percent and advantageously from about 20 to 75 weight percentbased on the potassium pyrosulfite and water. In amounts up to about 20weight percent water, damp crystals are provided and in amounts of about40 weight percent, for instance, a solution can be provided. When insolution, however, the potassium pyrosulfite is in the bisulfite form.

The crystals passed into tank 110 are dissolved in sufiicient water forease of handling. Generally, enough water is added to at least form aslurry which is pumpable, e.g., contains about 60 to 70% solids,although, if desired, sufiicient water can be added to form a solution.A ratio of solids to Water in dissolver tank 110 is advantageouslycontrolled by density means to a density control 103. Generally about 35to 55%, preferably about 40 to 50%, solids are in the solution which isremoved from dissolver 110 through line 116 by pump 118. Water issupplied to dissolver 110 in the form of condensate from the cooler 70via line 72 and via lines 74 and 76 from the steam ejectors 62 and 64,respectively. Fresh water can be added through line 120, if desired.

The potassium pyrosulfite in the presence of, or in contact with, wateris heated to temperatures sufficient to produce sulfur dioxide andtemperatures generally from about l to 225 F., preferably from about 150to 225 F. can be used under ambient pressure conditions.

The decomposition of the potassium pyrosulfite in contact with water canbe conducted under ambient pressures or superatmospheric pressures, forinstance from about 0 to 300 p.s.i.a., generally from about 15 to 150p.s.i.a. although ambient pressures can be advantageously employed. In amodification of this aspect of the present invention, superatmosphericpressures can be employed when the potassium pyrosulfite is in aqueoussolution to increase the concentration of the solution at highertemperatures, e.g. above about 230 F. and up to about 375 F., with aconsequent increase in the partial pressure of sulfur dioxide in thepotassium pyrosulfite and an enhancement of the production of sulfurdioxide. Generally the water employed in this aspect is from about 5 to70 weight percent. For instance, if potassium bisulfite solutions areheated under a pressure of about 100 p.s.i.a. with a temperature ofabout 350 F., the solubility of the pyrosulfite crystal is much greater,e.g. about 75 weight percent based on the crystals and water. Thishigher concentration provides a much higher partial pressure of sulfurdioxide than at lower concentrations and the percent conversion ofpotassium pyrosulfite to potassium sulfite is greater.

The utilization of water to enhance decomposition of the potassiumpyrosulfite is preferable to the anhydrous procedure since it obviatesthe desirability of using such inert materials and the expense ofhandling such mate rials and it provides for the production of sulfurdioxide at significantly lower temperatures and a higher conversion ofpotassium pyrosulfite (bisulfite) to sulfur dioxide.

Since there is generally a small amount of sulfur trioxide present inwaste gases containing sulfur dioxide, a small amount of potassiumsulfate is formed which is periodically removed. Additionally, oxygenpresent in the waste gases can react with the potassium sulfite toproduce additional potassium sulfate so that it may be desirable to addan oxidation inhibitor, e.g. organics such as hydroquinone, etc., to thepotassium sulfite solution. The potassiurn sulfate generallycrystallizes with the pyrosulfite and can be separated by periodicallyredissolving the pyrosulfite crystals, which are more soluble in waterthan potassium sulfate. The redissolved pyrosulfite crystals can berecycled to the reaction zone after the solid potassium sulfate isremoved. Potassium sulfate is desirable as a constituent of fertilizers.If desired, this process can be directed toward the production ofpotassium sulfate by omitting the inhibitor and increasing the amount ofpotassium sulfate produced.

The potassium bisulfite solution in line 116 is passed to stripper zone,designated generally as 125, where the solution is heated to atemperature, e.g. 230 to 300 F., sufficient for decomposition of thepotassium bisulfite to produce sulfur dioxide and a potassium sulfitesolution. The decomposition is advantageously accomplished in a multipleeffect stripping operation to increase the overall conversion ofpotassium bisulfite to potassium sulfite and reduce the energy, i.e.steam requirements of the system. As illustrated, stripper zone includesa stripping column 122 although, if desired, two or more effects may beused. The solution fed to column 122 through line 116 is heated by steamin heat exchanger 150 to a temperature, generally below the boilingpoint at the operating pressure of the column, sufficient to decomposethe potassium bisulfite in the solution to produce potassium sulfite,and sulfur dioxide and introduced into column 122 at this temperature.Although the bisulfite solution has been decomposed, there is no way forthe sulfur dioxide to be released until a driving force is provided.Accordingly, an inert gas, such as nitrogen, methane, argon, helium,etc., is introduced into column 122 through line to drive the S0 out ofsolution and preferably saturate the inert gas with S0 when the partialpressure of sulfur dioxide in the vapors within the column equals thepartial pressureof sulfur dioxide in the solution, no additional sulfurdioxide can be removed from solution regardless of the energy suppliedto the column. When using several columns, the energy or steamrequirements of the system are reduced by recovering energy from thevapors produced in each stripper column through the use of these vaporsto provide heat for the succeeding column. The inert gas containing S0is removed through line 124, heat exchanger 126 wherein the S0 iscondensed and passed to separator 128. The condensed S0 (containing anywater removed from column 122) is removed from the bottom of separator128 through line 138 and the inert gas through line 132. Blower 130 inline 132 returns the inert gas to column 122. The partially decomposedpotassium bisulfite solution is withdrawn from column 122 through line134 and passed by pump 136, to the absorber feed tank 106. Temperaturesof from about 230 to 300 F., and pressures of up to about 200 p.s.i.a.within the stripper column 122 are typical.

The solution in line 116 is generally heated to as high a temperature aspossible without producing potassium sulfate and without boiling thesolution. Heating increases the partial pressure of S0 in the solution,up to the boiling point of the solution, to provide a large pressuredifferential between the S0 in the solution and in the inert gas so thatthe inert gas can sweep the S0 from the solution as it passes throughcolumn 122. Generally, temperatures of about 230 F. up to about 600 F.,but preferably below 400 F. above which sulfate can form, and pressuresof about 0 to 300 p.s.i.a., preferably about 15 to p.s.i.a. can be usedin the decomposition zone. The inert gas is passed through the column ata suflicient velocity to strip S0 from the solution but insufficient toblow solution from the column, e.g. about 1.5 to 20 f.p.s., preferablyabout 2 to 8 fps. Since much of the water is condensed in the column atits operating conditions, little water vapor will be removed with theinert gases allowing re-use of the regenerated potassium sulfitesolution.

17 18 When it is desired to remove potassium sulfate, an unper hour andthe pH of the solution in the crystallizer is desirable by product ofthe process, from the potassium about 7.0 to 7.2. The system withoutbonnet 13 plugged bisulfite, a portion of the solution in line 116 canbe every two to four hours whereas with the bonnet plugging divertedthrough line 144 where a centrifuge 146, or, if has not occurred. Sulfurdioxide is recovered in the stripdesired, a filter, separates thepotassium sulfate crystals per by heating the feed line at 200 p.s.i.a.and 250 F. through discharge line 147. If a slurry of potassium biandpassing nitrogen through the column.

TABLE II Stripper Water teed Flue Into Flue Absorber Absorber Crystalcolumn to pregas absorber gas feed discharge discharge Filtrate feedscrubber Process stream line 12 30 bus 14 hne 16 line 18 blade 108 line102 line 116 line 24 S0 lbs/min 15. 15. 61 2. 49 13. 12 2. 49 K 8 0 lhsImin 13. 25 15. 27 4. 25 11. 0 4. 25 KzSO ,1bs./mjn 18 21 16. 78 0. 0916 7 0. 09 K 80 lbs./min 1.77 1.77 0.27 1 5 0. 27

Flue gas, a.c.f.m Fly ash, grains/it! G. .m 0.10 1. 0 86. 0

p Relative humidity, percent 6 1 8 l 1.5 grains per cubic foot into anelectric precipltator and 0.3 grain out of the electric precipltetor.

3 Out of absorber zone 30, the relative humidity was 83%.

sulfite is used as feed to stripper column 122, the diversion EXAMPLE Hand separation of potassium sulfate can be accomplished inline or line142. Essentially the same procedure used 1n Example I is f 11 The natureof the system and its Operation 1S such that 30 o owed except an aqueoussolution of cesium sulfite 1s it is not adversely affected by the cyclicnature of plant l if g gg gf gg sulfite to produce cesmm operations, thetemperature of gases to be processed (e.g.

drop in temperature of power plant gases in the evening), EXAMPLE HI orthe sulfur or solids content of the gas, and thus provides greatflexibility. Thus it can purify any gas containing S0 S0 or particulatesolids or any other similar types of components in any type of gas whichis compatible with the system and will not be deleteriously aflected by,or

Essentially the same procedure used in Example I is followed except anaqueous solution of rubidium sulfite is used instead of potassiumsulfite to produce rubidium bisulfite and recover S0 deleteriouslyaffect, the system. EXAMPLE IV The S0 recovery system described aboveand in the attached drawings was employed in the treatment of iiue g g ythe i 1 Example t gas from a coal-burning power plant. To evaluate thetemi 6 111m, argQIl as e perature drop of the incoming flue gas afterbeing routed gas m P ace 0 m rogen- It is claimed:

through the pre-scrubber, several runs were made at varyv ing ratios ofscrubbing water to waste gas. In each run li g g zzji fg g 223125 23252? fi gggggg: the i gg i i g the i 2: a 2:: tion zone, contacting theSo -containing gas in the abfietfaglrjacfmgbout 3561 11cgieptzrnrgpiureagnd hla y S0l1'ptl0(l11 fzrone with an aqueous solutionof metal sulfite of the gas as it exited the top of the impingementtarget Se e-cte 3 the i conslstmg of Potassium sulfite M I b 1 5 cesiumsu te, and ru ldllll'n sulfite at a temperature sufwas recorded. Resultsare presented 11]. Ta e e ow. fluent to absorb S02 from the gas andproduce an TABLE I aqueous solution of the corresponding bisulfite whichis water a precursor of S0 removing the precursor-containing flow toMolsoi Flue gas solution from the absorption zone, precipitating the bi3 sulfite as crystals of the corresponding metal pyrosulfite g.p.m. drygas temp.,F. percent having a purity of greater than about separating(10 04060 300 6 said crystals, adding sufiicient water to the crystalsto 0.1 0.075 255 8 form a pumpable mixture, heating the mixture to a. 88: a? temperature and pressure below the boiling point of the 300 0.135128 100 0 mixture and suflicient to decompose the bisulfite which.

may be in the form of pyrosulfite to produce S0 and EXAMPLE I an aqueoussolution of sulfite, passing a gas inert to the The following table,Table H, sets forth a specific exm tu e through the mixture incountercurrent contact ample of this process within the conditions andparameters 111 a pp Zone In an amount alldflt a velocity sufset fo th ith dis s ion ref i to FIGS, 1 and 2 f 65 ficient to strip S0 from theheated mixture, condensing the drawing and using essentially the samespecific ex S0 from the inert gas to recover it and conducting theamples noted in the drawings, the table giving specific inert gas backto the pp g Zonecompositions of the streams for the various flow linesand 2. Process of claim 1 wherein the pumpable mixture prevailingtemperatures. The flue gas has the typical comis a slurry containingabout 60 to weight percent solids. position in mole percent; sulfurdioxide, 0.3%; oxygen, 70 3. Process of clalm 1 wherein the p pablemlXture 3.4%; water vapor, 60%; carbon dioxide, 14.2%; nitrois asolution containing about 35 to 55 weight percent gen, 76.1%; and sulfurtrioxide, 0.0003%. Essentially no fite. potassium sulfate is produced.The vacuum type flash 4. Process of claim 1 wherein the inert gas isselected cooler is operated at l p.s.i.a. and 104 F., water is refromthe group consisting of nitrogen, helium, methane moved from the flashcooler at a rate of about 140 pounds and argon.

5. Process of claim 1 wherein the mixture is heated to about 230 to 600F. at a pressure of about to 300 p.s.i.a.

6. Process of claim 4 wherein the mixture is heated to about 230 to 400F. at a pressure of about 0 to 300 p.s.i.a.

7. Process of claim 6 wherein the pumpable mixture is a slurrycontaining about 60 to 70 weight percent solids.

8. Process of claim 6 wherein the pumpable mixture is a. solutioncontaining about 35 to 55 weight percent bisulfite.

9. Process of claim 1 wherein the sulfite is potassium sulfite, thebisulfite is potassium bisulfite, the crystals are potassium pyrosulfiteand the pumpable mixture contains 1 to 99 weight percent water basedupon the potassium pyrosulfite and water.

10. Process of claim 9 wherein the mixture contains about to weightpercent water.

'11. Process of claim 10 wherein the purity is above weight percent.

'12. Process of claim 1 wherein the purity is above 90 weight percent.

References Cited UNITED STATES PATENTS 6/1926 Eustis 23-178 R 1/1945Rosenstein 233 OTHER REFERENCES OSCAR R. VERTIZ, Primary Examiner C. B.RODMAN, Assistant Examiner US. Cl. X.R. 423539

